In driving of printhead mounted in an inkjet printing apparatus, it is necessary to accurately control energy applied to the printhead, since a change of amount of ink discharged from an inkjet printhead (hereinbelow, referred to as a “printhead”) may cause density unevenness in a printed image or variation of image quality due to individual difference of printing apparatus. Further, in a case where the driving energy applied to the printhead is insufficient, ink discharge failure may occur, or in a case where the energy is oversupplied, the life of the printhead may be shortened.
Accordingly, the accuracy of printhead driving voltage must be suppressed to about ±1% of rated voltage.
Generally, to set an output voltage in a power circuit, a semiconductor band-gap voltage is used as a reference voltage. As the accuracy of the band-gap voltage is about ±2%, to realize the ±1% accuracy required in the driving power supply, conventionally the output voltage is controlled by using a variable resistor or the like during manufacturing process of a printhead driving power supply circuit.
On the other hand, the printhead, having a structure removable from the printing apparatus main body, is generally manufactured separately from the printing apparatus main body.
For example, in a thermal inkjet printer in which electric energy is applied to electrothermal transducers provided around ink channels to cause heat and to discharge ink with bubbles formed by the heat, even though the same driving voltage and the same driving pulse are applied, a constant ink discharge amount cannot be obtained due to manufacturing variations in resistor values of the electrothermal transducers and/or the thickness of insulating films between the electrothermal transducers and an ink chamber.
Accordingly, the variations in the manufacturing process are reduced by conducting an ink discharge test upon manufacturing and controlling the driving voltage to attain a constant discharge amount.
Recently, an optimum driving condition is measured in an ink discharge test conducted upon manufacturing a printhead, and this condition is set for the printhead (See Japanese Patent Application Laid-Open No. 8-118628).
However, the above conventional art has the following problems.
(1) Generally, the driving power circuits installed in the printhead and the printing apparatus are separately manufactured. Upon manufacturing the printhead, accurate ink discharge measuring test is conducted, and an optimum driving condition is set for each printhead. Accordingly, accurate voltage control is made in the power circuit used in the test device. On the other hand, upon manufacturing the driving power source of the printing apparatus main body, a process of controlling the accuracy of output voltage to about ±1% of rated voltage is required. That is, control processes are required in the respective power circuit of the printhead test device and the driving power circuit of the printing apparatus main body to ensure the absolute voltage accuracy. Further, as high-quality components must be used as constituent parts of the power circuits, the total costs are increased.
(2) The setting accuracy of driving voltage in the test device used upon measuring a printhead driving condition and that of driving power of the printing apparatus main body, respectively within 1% to 1.5% variations, must be tolerated for the sake of practical use. Accordingly, there is an relatively 2% to 3% error between the voltages set in these power circuits. For this reason, upon designing a printhead, the driving condition must be designed in consideration of the error, and as a result, energy may be oversupplied to the printhead in a commercially practical product. Such energy oversupply is undesirable in view of the life span of printhead.